<html>
<head>
  <style type="text/css">
   .box {
      padding: 10px;
      padding-bottom: 0;
      padding-right: 0;
      display: flex;
      flex-direction: column;
      height: 800px;
      width: calc(100vw - 20px);
      background-color: #000;
    }
    .content {
      display: flex;
      margin-bottom: 10px;
      flex: 1
    }
    .b {
      flex: 1;
      margin-right: 10px;
      background-color: yellow;
    }
  </style>
</head>
<body>
  <div class="box">
    <div class="content">
      <div class="b">1</div>
      <div class="b">2</div>
      <div class="b">3</div>
    </div>
    <div class="content">
      <div class="b">4</div>
      <div class="b">5</div>
      <div class="b">6</div>
    </div>
    <div class="content">
      <div class="b">7</div>
      <div class="b">8</div>
      <div class="b">9</div>
    </div>
  <div>
</body>
</html>
